Solid solutions, applicable as catalysts, with perovskite structure comprising noble metals

ABSTRACT

Materials with a perovskite structure in form of solid solutions with general formula: A z Zr 1 — x B x O 3  Where A is Ba or a rare earth element, B is Pt, Ir, Rh or Ce z is 1 when A is Ba and is ⅔ when A is a rare earth, x is in the range 0.01 and 0.8.

[0001] This invention concerns solid solutions with a perovskitestructure comprising noble metals, which are useful as catalysts incombustion reactions and, in general, in oxidation processes at hightemperature (production of syngas, olefins, elimination of VOC andunburned emission from motor-vehicles).

[0002] Perovskites are ceramic materials formed by the combination ofmetallic elements with non-metallic elements (usually oxygen) placed ina certain crystalline structure. Their name derives from the specificmineral ‘Perovskite’ (CaTiO₃). From a technological point of view,Perovskites are of considerable interest because the single crystallinestructure can exhibit a wide range of properties.

[0003] In their ideal structure perovskites have a general formula ABX₃and consist of cubes composed of two metallic cations (A & B) and anon-metallic anion (X) in the ratio 1:1:3. In the cubic cell cations Aare bigger and are co-ordinated with twelve X anions, while cations B,which are smaller, are co-ordinated with six X anions (FIG. 1).

[0004] The high symmetry of the atomic disposition imposes someconstraints on the dimension of the ions within the structure, as shownby Goldschmidt [1]; it is therefore important to bear in mind thedimension of the element in the various oxidation states and in thedifferent co-ordination numbers Table 1) [2]. TABLE 1 [2] IonCo-ordination number Ionic radii (pm) Ba⁺² 6 149 Ba⁺² 12  175 Ce⁺³ 6 115Ce⁺³ 12  148 Ce⁺⁴ 6 101 Ce⁺⁴ 12  128 Pd⁺² 6 100 Pd⁺³ 6 90 Pd⁺⁴ 6 75.5Zr⁺⁴ 6 86 O⁻² — 121 Rh⁺³ 6 80.5 Rh⁺⁴ 6 74 Rh⁺⁵ 6 69

[0005] Inspection of the scientific and patent literature shows thatcurrent syntheses very often involve a rare earth element in position A,and a transition element (such as Fe, Mn, Co, Ni, Cr) in position B, andthat very few perovskites containing noble metals in a high oxidationstate have been synthesised up to now.

[0006] Materials with a perovskitic structure having the general formulaA_(z)Zr_(1−x)B_(x)O₃ are now found where:

[0007] A is Ba or a rare earth element;

[0008] B is Pt, Ir, Pd, Rh, or Ce;

[0009] z is 1 when A is Ba and ⅔ when A is a rare earth element;

[0010] x is in the range 0.01 to 0.8;

[0011] A is usually Ba or La (more usually Ba);

[0012] B is usually Pd, Rh or Ce.

[0013] The materials according to this invention in which B is Pd or Ceare particularly suitable as catalysts for the catalytic combustion ofmethane for power application. Catalysts based on supported Pd arecurrently the only ones displaying a catalytic activity for methanecombustion high enough to light-off the reaction at low temperatureinlet conditions, low contact times and lean fuel concentrationscharacteristic of modern gas turbines fed with natural gas. Anotherfavourable characteristic of these systems is the negligible volatilityof the various species of Pd (metal, oxide, hydroxide) below 1000° C.

[0014] The catalysts based on supported Pd of the current technologydisplay a complex hysteresis cycle which transforms the Pd into ametallic state which is catalytically inactive at high temperatures,with a further re-oxidation to PdO at lower temperatures (FIG. 2) [3].

[0015]FIG. 2 [4] shows this hysteresis cycle for a typical catalystbased on Pd supported on alumina.

[0016] The catalyst 10 heated to 980° C. (curve 1), cooled to 200° C.(curve 2), heated again to 980° C. (curve 3) and cooled to 200° C.(curve 4). All the steps were carried out at 5° C./min.

[0017] The catalyst subjected to an air flow starts loosing weight attemperatures of around 400° C.; this weight loss is due to the loss ofwater chemisorbed on the surface. At temperatures higher than 800° C.the rate of weight loss is suddenly increased due to the transformationof PdO to Pd, which starts at this temperature and which is completed at970° C. During a first cooling in air the sample only starts to gainweight below 570° C. and down to 380° C., but not all the weight lost inthe previous step is recovered. When the sample is heated again a smallweight loss is observed between 700° C. and 980° C., with a further gainduring the cooling step similar to that of the previous cycle.

[0018] A certain degree of reduction of Pd together with its oxidisedspecies is desirable because the first catalytic step involvesdissociation of the C—H bond, which occurs on reduced species, whereasthe further oxidation steps occur on the oxidised species [5].

[0019] For the above reasons it could be advantageous to utilisepalladium in a high oxidation state that is fully reducible to themetallic state only at temperatures higher than those foreseen for anindustrial operation (≅1300° C.).

[0020] This goal has been realised with the present invention, whichallows for the insertion of palladium in a high melting perovskite ofthe BaZr_(1−x)Pd_(x)O₃ type. Attention has been drawn in a previouspatent (6) to the high melting point (≅2600° C.) of the systems with aperovskite structure based on BaZrO₃.

[0021] In a similar way, the present invention concerns BaZrCeO₃ systemsin order to take advantage of the well known oxidising properties ofCe⁺⁴.

[0022] One of the favourable characteristic of the barium-based systemsis the limited production they permit of NO_(x): recent studies haveshown that barium has the ability to decompose NO_(x) to N₂ and O₂, sothat the barium-based systems proposed here may be of generalapplicability for all the combustion processes aiming at limiting NO_(x)emission—including those for motor-vehicles.

[0023] The present invention allows for the substitution of at leastpart of the barium with a rare earth, such as lanthanum, in order toobtain systems which are strongly weight-stable at high temperatures.Catalysts containing large amounts of barium could pose problems at veryhigh temperatures (as a result of the high volatility of barium) unlessthe barium is combined within the perovskite structure. The possibilityof using perovskites composed of barium and a rare earth could furtherdiminish such risk.

[0024] The materials of the present invention for which B is Rh areparticularly suitable as catalysts for the partial oxidation of methaneto syngas (CPO).

[0025] The two main technologies for the production of syngas are steamreforming of methane or virgin naphtha, and the non-catalyticautothermal processes.

[0026] Steam reforming involves as a first step, after the eliminationof the sulphur containing compounds, the use of large scale catalyticreactors, prone to the formation of carbon, and with complex problems ofdownstream heat recovery.

[0027] The non-catalytic autothermal processes, on the other hand,involves very high temperatures in order to avoid the formation ofcarbon. As a consequence the use of a O₂/CH₄ ratio higher than thestoichiometric value and equal to about 0.7 becomes necessary, leadingto the undesired formation of H₂O and CO₂ which reduces the efficienciesof subsequent syntheses.

[0028] Various solutions have been proposed in order to overcome theabove drawbacks, among which catalytic partial oxidation appears to beone of the most promising for the following reasons:

[0029] 1) It involves carrying out the oxidation reaction, CH₄+0.5O₂→CO+2H₂, at oxygen concentrations close to stoichiometric, and atlower temperatures (around 800-900°) thereby resulting in greater syngasyields, both in respect of methane and oxygen;

[0030] 2) The oxidation reactions are very fast, involving very highspace-velocities; yields are high, with contact times of the order ofmilliseconds: the reactors may therefore be very small;

[0031] 3) The partial oxidation reaction leads to a production H₂/COratio equal to 2, and therefore more suited to both Fischer-Tropsch andmethanol syntheses;

[0032] 4) The process is very fast, and being catalytic makes itpossible to control better the carbon formation.

[0033] The most promising catalysts for the above goals are thosecontaining rhodium in a high melting and non-acidic matrix, such asthose of the BaZr_(1−x)RhxO₃ systems proposed here.

[0034] The materials of the present invention may be prepared withsuitable modifications to the citrate method described in [6].

[0035] The citrate route is a wet method for the synthesis of mixedoxides, which was proposed by Delmon and co-workers in the late nineteensixties as an alternative to co-precipitation and to the ceramic methodfor the manufacture of high tech ceramic materials and catalysts [7, 8,9, 10, 11, 12].

[0036] This method offers a number of advantages, in particular it makesit possible to obtain:

[0037] mixed oxides over a wide range of composition;

[0038] good control of the stoichiometry;

[0039] an excellent interspersion of the elements in final product;

[0040] very small grain size materials.

[0041] The first step of the proposed preparation method consists of thepreparation of an aqueous solution of the nitrates of the requiredmetals with citric acid (in a ratio of 1 equivalent of citric acid perequivalent of cation) and if necessary ammonium hydroxide.

[0042] The solution obtained is then concentrated by evaporation in arotavapor and dried under vacuum until a meringue-type spongy solid isobtained, which may easily be ground to an amorphous powder.

[0043] Calcination then follows, which eliminates the organic substanceand yields the desired oxides: a microcrystalline solid is obtained,with the ions well interspersed, often in a monophasic system.

[0044] The starting salts generally used in the original method werenitrates, because of their good solubility. However problems arise withthese solutions during concentration, drying and calcination, due to theevolution of nitrogen oxides. Nitrogen oxides, in addition to beingtoxic and corrosive to the materials of the oven, may lead to a suddendecomposition of the organic substance, possibly resulting in anexplosion or fire hazard. This occurs particularly when cations arepresent (such as those of Mn, Fe, Co, Cu and Ag), which may catalyse theoxidation of the organic substance.

[0045] The proposed method (anticipated in previous patents [6, 13, 14,15]) has now been modified and improved in order to make it more widelyapplicable and less hazardous.

[0046] In the new method:

[0047] nitrates are not used as starting salts, particularly in thepresence of elements displaying a high catalytic activity for thecombustion of the organic materials;

[0048] the decomposition is carried out in milder conditions than in theoriginal citrate route, thus involving: a lean oxygen gas flow (1.5% O₂)and a low temperature (T˜350° C.).

[0049] The method described in this invention combines the advantages ofthe wet methods with the possibility of utilising readily availablereagents which are among the cheapest for the elements to be complexed;they are also easy to handle during the preparation, particularly interms of temperature control.

[0050] The process envisages the following steps:

Preparation of the Solution

[0051] A clear solution containing the required elements is preparedusing citric acid and ammonium hydroxide. The preparation then involvessome special features which may be essential for achieving the finalresult. For example: as complexation is usually favoured by lowtemperatures, because of the low energy of activation in concentratedsolutions, the use of externally ice-cooled solutions is preferred tofavour the complexation of the cations and to reduce evaporation ofammonia. The dissolution of the noble metals, in particular palladium,is assisted by the presence of oxidising substances: in view of this, ifthe synthesis makes use of barium, for example, it may be convenient touse BaO₂, otherwise H₂O₂ may be used. For the preparation of solutionscontaining Zr it is possible to use Zr isopropoxide (in an isopropanolsolution) or hydrated zirconia. If Zr isopropoxide is used, it isnecessary to carry out its hydrolysis by boiling it in a citric acidsolution for a few hours until a clear solution is obtained

Concentration and Drying of the Overall Solution

[0052] The concentration may be carried out in a rotavapor. The viscousmaterial that is obtained after this operation is then dried in a vacuumoven, typically at up to 200-220° C., in order to obtain a solid with ameringue-like consistency. This solid is then crushed and sieved inorder to obtain a fine powder—with particle dimensions lower than, say0.4 mm (100 mesh).

[0053] The initial overall solution could also be spray-dried, ideallyutilising a fluid, such as CO₂, under supercritic conditions;alternatively it may be employed for the impregnation of a support, suchas silica or alumina, in order to produce a supported catalyst.

[0054] Decomposition of the Organic Substance

[0055] The powders obtained from the previous step contain a highpercentage of organic material which should be decomposed by oxidation.Best results are obtained utilising mild conditions, involving, forexample, a flow of N₂ containing 1.5% O₂. The decomposition starts ataround 330 to 390° C., and the progress of the reaction can be monitoredeither by a continuous measure of the powder temperature or by computingthe oxygen consumption from the measured oxygen concentration in theoutlet flow from the reactor. The powders at this stage contain mainlyan amorphous phase characterised by a good interspersion of theelements.

[0056] They may also contain a small percentage of carbon, particularlyin form of carbonates.

Final Calcination

[0057] A further step of calcination at high temperature (up to, say,800-1000° C.) is then performed in order to attain full crystallisationof the powders (11). The powders so obtained are of reliablestoichiometry and, in contrast to what occurs with other technologies,free of impurities.

[0058] In conclusion, the proposed variant of the citrate method of thisinvention allows for the preparation of aqueous solutions in citric acidand ammonia of very many elements of the periodic table without makinguse of nitrate salts. The interspersion in solution at atomic scaleprovides the best pre-condition for a good interspersion of the driedpowder and, eventually, of the calcined powder, because thedecomposition of the organic part is performed under mild conditionswith good temperature control and in the absence of nitrates.

[0059] The following examples illustrate the invention in a greaterdetail.

EXAMPLES 1-3 Preparation of BaZr_(1−x)Pd_(x)O₃, BaZr_(1−x)Rh_(x)O₃,Ba_(1−x)La_(2/3x)ZrO₃

[0060] The following reagents were used, in quantities reported intables 2, 3, 4, 5, 6, 7, 8:

[0061] Zirconium isopropoxide solution, Zr(C₃H₇O)₄, in isopropyl alcohol(20.4% Zr b.w.), with density 1.044 g/cm³, (Aldrich);

[0062] Citric acid monohydrate, C₆H₈O₇*H₂O, (99.8% b.w.), (Carlo Erba);

[0063] Ammonium hydroxide, NH₄OH, (25% NH₃ b.w.), with density 0.91g/cm³, (Merck);

[0064] Barium peroxide, BaO₂, (92.66% b.w., the rest being BaO),(Materials Research, MRC);

[0065] Palladium II acetate, Pd(C₂H₃O₂)₂, (48.11% Pd b.w.), (Chempur);

[0066] Rhodium II acetate (36.59% Rh b.w.) (Reacton);

[0067] Lanthanum III Acetate, La(OOCCH₃)₃*1.5H₂O (Reacton).

EXAMPLE 1 BaZr_(1−x)Pd_(x)O₃ Preparation

[0068] Two separate solutions are prepared and then mixed together: onecontaining dissolved zirconium and the other barium. Palladium is mixeddirectly into the solution containing barium in order to facilitate thedissolution: as indicated above, the oxidising properties of bariumperoxide. allow for an easy dissolution of palladium. This reduction inthe number of solutions to be prepared results in lower costs in theevent of case of scale-up of the process. TABLE 2 Quantities of thereagents to be used for the zirconium solution for obtaining 20 g offinal catalyst. Zr(C₃H₇O)₄ Deionised NH₄OH % Pd (g) H₂O (mL) Citric acid(g) (mL) 0 32.33 100 30.84 22 5 28.18 93 25.75 22.5 10 23.47 84 21.5 1915 19.04 68 17.4 13 20 14.61 52 13.3 20 25 10.17 36 9.3 7.1 36.5 0 0 0 0

[0069] TABLE 3 Quantities of the reagents to be used for the barium andpalladium solution for obtaining 20 g of final catalyst. BaO₂DeionisedH₂O Citric NH₄OH Pd(C₂H₃O₂)₂ % Pd (g) (mL) acid (g) (mL) (g) 012.13 120 34.7 15 0 5 12.07 118.5 34.15 40 2.08 10 12.01 118 34 40 4.2215 11.90 117 37.4 42 6.24 20 11.84 116 33.6 53 8.32 25 11.75 116 33.4 6010.40 36.5 11.55 120 38.4 102.5 15.18

[0070] The first to be dissolved is zirconium. The citric acid solutionis added to the zirconium isopropoxide, the products of hydrolysis beingkept boiling under vigorous stirring conditions. The dissolution occursin about eight hours. The iced-cooled ammonium hydroxide solution isthen added to the zirconum solution, also previously ice-cooled.

[0071] The dissolution of barium peroxide requires a strong excess ofcitric acid compared to that proposed for the traditional citrate route.

[0072] A citric acid solution is prepared in the quantity necessary forthe dissolution of both barium and palladium. Barium peroxide is slowlyadded with stirring to the citric acid solution at room temperature inorder to minimise the formation of lumps. A slow and controlleddevelopment of small bubbles of oxygen is observed. As soon as thebarium is dissolved, palladium acetate is added. The vessel is thenice-cooled and the calculated amount of ammonium hydroxide solution isadded.

[0073] A transparent solution is obtained which is added to the coldzirconium solution previously prepared. The solution is thenconcentrated in a rotavapor.

[0074] The preparation described above is carried out for all theBaZr_(1−x)Pd_(x)O₃ samples.

EXAMPLE 2 BaZr_(1−x)Rh_(x)O₃ Preparation

[0075] TABLE 4 Quantities of the reagents to be used for the zirconiumsolution for obtaining 20 g of final catalyst. Zr(C₃H₇O)₄ DeionisedNH₄OH % Rh (g) H₂O (mL) Citric acid (g) (mL) 2.5 30.137 115 36.4 86.4 527.808 104 33.2 86.4 10 23.51 84 21.5 19 18.58 14.22 72 17.82 38

[0076] TABLE 5 Quantities of the reagents to be used for the barium andrhodium solution for obtaining 20 g of final catalyst. BaO₂ DeionisedH₂OCitric NH₄OH Rh(II) acetate % Rh (g) (mL) acid (g) (mL) (g) 2.5 12.059125 33.4 86.4 1.3665 5 12.092 125 33.3 86.4 2.7330 10 12.023 118 34 405.4659 18.58 10.476 133 33.15 131 9.3162

[0077] The zirconium solution is prepared in a similar way to thatdescribed for the palladium containing systems. The barium containingsolution is also prepared in a similar way, and the citric acid computedfor the complexation of rhodium is directly added to the citric acidcomputed for barium, as before. Rhodium acetate followed by ammoniumhydroxide are then added to the solution containing barium. Thetransparent solution so obtained is then concentrated in a rotavapor.

EXAMPLE 3 Ba_(1−x)La_(2/3x)ZrO₃ Preparation

[0078] TABLE 6 Quantities of the reagents to be used for the zirconiumsolution for obtaining 20 g of final catalyst. Zr(C₃H₇O)₄ DeionisedNH₄OH X (g) H₂O (mL) Citric acid (g) (mL) 0.25 32.277 168.9 42.226135.15

[0079] TABLE 7 Quantities of the reagents to be used for the barium andrhodium solution for obtaining 20 g of final catalyst. BaO₂ DeionisedH₂OCitric X (g) (mL) acid (g) NH₄OH (mL) 0.25 9.499 95 23.752 101.36

[0080] TABLE 8 Quantities of the reagents to be used for the barium andrhodium solution for obtaining 20 g of final catalyst. La(OOCCH₃)₃ *Deionised Citric NH₄OH X 1.5H₂O H₂O (mL) acid(g) (mL) 0.25 4.5109 163.958 8.45

[0081] The solution containing zirconium is prepared in a similar way tothat described for the palladium and rhodium containing systems. Thesolution containing barium is also prepared in a similar way. Thesolution containing lanthanum is then prepared; this solution is bestobtained using the crystalline form of lanthanum acetate containing 1.5moles of water (as for the reagent previously proposed). To obtain thissolution the citric acid solution is ice-cooled and added withice-cooled ammonium hydroxide solution and then with the lanthanumacetate. A transparent solution is obtained in a few minutes. The threesolutions containing zirconium, barium and lanthanum are mixed togetherand the resulting solution is concentrated in a rotavapor. The operatingconditions for this step, which lasts for about an hour are:

[0082] Temperature≅80° C.;

[0083] Pressure

initial≅250 mbar; final 50≅mbar;

[0084] Revolution speed=100 rpm.

[0085] The products obtained from the rotavapor are very viscoussolutions with a honey-like consistency; drying is completed undervacuum, in a vacuum oven.

[0086] The vacuum is connected via a liquid nitrogen trap to eliminateammonia and other light substances coming from the sample. Vacuum isobtained with a double stage pump (RC5 Vacuubrand).

[0087] Drying has been performed with the following thermal program:

[0088] Temperature rise from ambient to 50° C. in 15 minutes;

[0089] Temperature rise from 50° C. to 200° C. in 50 hours;

[0090] Dwell at 200° C. for 10 hours;

[0091] Temperature decrease to ambient (uncontrolled speed).

[0092] The dried samples have a meringue-like consistency, and areeasily ground and sieved through a 100 mesh sieve. The powders are thenput in a tubular quartz reactor 5 cm i.d. and decomposed in a fluidisedbed. The inlet gas flow to the reactor (of 94% N₂ and 6% air) ismaintained at a flow rate of 120 l/h.

[0093] The treatment in the fluidised bed is performed in two stages. Inthe first stage, at 330° C.-380° C., most of the organic matter isdecomposed. When the combustion appears to be almost ended (judged bythe lowering of the powder temperature) the air content of the feed gasmay be gradually increased until it becomes solely air; this is achievedwithout altering the total gas flow rate. Calcination of the samples isthen performed with the following programme:

[0094] Heating from 380° C. to 500° C. at a rate of 2° C./min;

[0095] Dwell at 500° C. for 4 hours;

[0096] Lowering of the total flow from 120 L/h down to 60 L/h;

[0097] Heating from 500° C. to 800° C. at a rate of 2° C./min;

[0098] Dwell at 800° C. for 10 hours;

[0099] Cooling down to 25° C. in 4 hours.

EXAMPLE 4 Preparation of the BaZr_(1−x)Ce_(x)O₃ Samples

[0100] The following reagents were used, in quantities reported inTables 9 and 10:

[0101] Zirconium isopropoxide solution, Zr(C₃H₇O)₄, in isopropyl alcohol(20.4% b.w.), with density 1.044 g/cm³, (Aldrich);

[0102] Citric acid monohydrate, C₆H₈O₇*H₂O, (99.8% b.w.), (Carlo Erba);

[0103] Ammonium hydroxide, NH₄OH, (25% b.w. of NH₃), with density 0.91g/cm³, (Merck);

[0104] Barium peroxide, BaO₂, (92.66% b.w., the rest being BaO),(Materials Research, MRC);

[0105] Cerium acetate (III) tetrahydrate, Ce(C₂H₃O₂)₃*4H₂O, (99.9%b.w.), (Chempur);

[0106] Two separate solutions are prepared and then mixed together: onecontaining dissolved zirconium, the other barium and the cerium. TABLE 9Quantities of the reagents to be used for the zirconium solution forobtaining 20 g of final catalyst. Zr(C₃H₇O)₄ Deionised Citric acid NH₄OHCe (g) H₂O.(mL) (g) (mL) 6.48 27.46 137.6 34.40 110.16 12.77 22.74113.95 28.49 91.23 24.77 13.73 68.79 17.19 55.06 43.05 0 0 0 0

[0107] TABLE 10 Quantities of the reagents to be used for the barium andcerium solution for obtaining 20 g of final catalyst. Deionised CeriumBaO₂ H₂O Citric acid NH₄OH acetate % Ce (g) (mL) (g) (mL) (g) 6.48 11.88131.51 30.19 123.74 3.60 12.77 11.61 136.26 29.51 120.96 7.09 24.7711.10 145.35 28.22 115 13.76 43.05 10.33 159.20 26.25 107.58 23.91

[0108] The zirconium solutions are prepared in a similar way for thatdescribed for the BaZr_(1−x)Pd_(x)O₃, BaZr_(1−x)Rh_(x)O₃,Ba_(1−x)La_(2/3x)ZrO₃ systems.

[0109] Cerium acetate is added to the aqueous solution of citric acid inthe quantity necessary for the complexation of both cerium and barium;the resulting product, after stirring for about 16 hours at roomtemperature, resembles lean yoghurt both in appearance and consistency.By adding barium peroxide the colour is changed to light brown, andsmall gas bubbles develop.

[0110] After mixing for about ten minutes and cooling in an ice andwater bath, ammonium hydroxide is added. A transparent, dark brownsolution is obtained. The solution containing zirconium is then addedand the overall solution so obtained is mixed for about half an hour,after which a perfectly clear, dark orange solution is obtained, whichmay be concentrated in the rotavapor. The various steps of concentrationin the rotavapor, drying in the vacuum oven, decomposition in thefluidised bed and calcination are similar to those for the othersamples.

EXAMPLE 5 Characterisation of the BaZr_(1−x)Pd_(x)O₃ Samples

[0111]FIG. 3 shows the XRD diffractograms of various BaZr_(1−x)Pd_(x)O₃samples calcined at 800° C. for 4 hours.

[0112] It can be seen that the sample corresponding to the stoichiometryof BaPdO₃ (i.e. with 36.6 Pd b.w.) shows a diffractogram that is ratherdifferent from the others. This sample, when calcined at highertemperatures, does not form the perovskite phase; this confirms that thepresence of a certain quantity of zirconium in the oxide is necessary inorder to obtain this phase—as indicated in the first claim.

[0113] The various phases were identified by the search-match method(JCPDS data base) while the phase compositions and cell parameters weredetermined with great accuracy by full profile fitting refinement(Rietvield method) (16), using the Hill&Howard procedure (17), theWYRIET program and the structural data necessary from ICDS (18), thusobtaining the quantitative phase compositions shown in table 11. TABLE11 Pd BaZr_(1−x)Pd_(x)O₃ BaCO₃ weight weight % x Calcination weight % %PdO weight % 5 0.1309  800° C. (air) 100 — — 10 0.2637  800° C. (air)100 — — 15 0.3985  800° C. (air) 96.5 3.5 — 15 0.3985 1200° C. (air)92.5 — 7.5 20 0.5352  800° C. (air) 91 3   6  

[0114] Cell parameters of the perovskite phase were determined, to forthdecimal place precision, for samples containing palladium from 0 to 20%b.w. Table 12 gives the values found. TABLE 12 Pd Crystal size weight %x Calcination A [Å] [Å] V [Å³] 0 0  800° C. (air) 4.1830(1) 360 73.19 50.1309  800° C. (air) 4.1799(1) 220 73.03 10 0.2637  800° C. (air)4.1675(1) 305 72.38 15 0.3985  800° C. (air) 4.1655(1) 190 72.28 150.3985 1200° C. (air) 4.1612(1) 245 72.05 20 0.5352  800° C. (air)4.1540(1) 145 71.68

[0115] The unusually precise determination of the cell parameters, dueto an excellent fitting of the experimental data, shows unequivocallythat the cell is perfectly cubic, with zirconium and palladiumcompletely randomly distributed among the sites B of the perovskite. Thesteady decrease of the cell parameter with the increase of the noblemetal content inside the structure, as shown in the previous table, andin FIG. 4, indicates that the noble metal is present in a high oxidationstate, probably as Pd⁺⁴ Indeed it should be recalled that only Pd⁺⁴ inoctahedral co-ordination presents an anionic radius (75.5 pm) smallerthan the ionic radius of Zr⁺⁴ in the same co-ordination (86 pm).

[0116] The presence of palladium within the structure in a highoxidation state reveals its lesser tendency to be reduced to themetallic state, with consequential deactivation, as seems to beconfirmed by the high temperature thermogravimetric data.

[0117] It is possible to maximise the surface area of the samples bydrying them under vacuum and low water partial pressure conditions at atemperature of up to about 200° C. This can be done either by employingvery long drying times or by periodically purging the vacuum oven withdry air (by, say, introducing an air flow at regular interval, followedby full vacuum periods).

[0118] Catalytic activity data for the combustion of methane have beenobtained for samples for which the surface areas have yet to beoptimised. To perform these tests, 0.4 g of catalyst are mixed with 0.8g of quartz, (140-200 mesh) and placed inside a quartz microreactor withinternal diameter 8 mm. A layer of quartz particles, within the 20-30mesh range and 12 cm thick, are then placed above the catalytic bed.

[0119] A gaseous, constant composition stream is fed to the reactor at aconstant flow rate, and the appropriate temperature-time profile isapplied:

[0120] Inlet Gas Composition

[0121] Methane: 1%, Oxygen: 4%, Nitrogen: 95%;

[0122] Flow

[0123] 24 L/h

[0124]FIG. 5 reports the temperature at which 20% conversion is reachedin runs employing both a gradually increasing and a gradually decreasingtemperature.

[0125]FIGS. 6 and 7 show the results of thermogravimetric analysis (TGA)performed on the 15% b.w. Pd sample. It is noteworthy that the firstonset of reduction of palladium occurs at about 100° C. higher than inthe following cycles in which PdO supported on the perovskite should bepresent. In other words it confirms that the perovskite stabilizespalladium by making its reduction more difficult than in simplysupported palladium.

[0126] The BaZr_(0.6015)Pd_(0.3985)O₃ sample (15% b.w. of Pd) wascalcined at 1200° C. in an oven operating with static air. Table 1 givesthe quantitative phase composition measured and Table 2 the cellparameter (a) and cell volume (V). It shows that only about 50% of thepalladium is extracted from the perovslcite and transformed at roomtemperature into PdO. This means that the other half of palladium isstill within the perovskite structure at 1200° C. The cell parameter issignificantly increased with respect to the same sample calcined at 800°C., in agreement with the decreased content of palladium within theperovskite phase.

EXAMPLE 6 Characterisation of the BaZr_(1−x)Ce_(x)O₃ Samples

[0127]FIG. 8 shows the diffractograms obtained on the different samplesof BaZr_(1−x)Ce_(x)O₃ with different weights of cerium, calcined at 800°C. for 10 hours. It can be observed that the greater the cerium contentin the sample, the less intense the characteristic peaks of theperovskite phase, which are also shifted to lower angles, indicating anincrease in cell volume. Table 13 reports the quantitative phasecomposition for all samples. TABLE 13 BaZr_(1−x)Ce_(x)O₃ BaCO₃ CeO₂% Ceweight % X weight % weight % weight % 6.48 0.1309 85 15 — 12.77 0.263779 21 — 24.77 0.3985 77 23 — 43.05 0.5352 77 10 13

[0128] Table 14 shows the cell parameters found for the differentsamples synthetised. TABLE 14 Ce weight % X a [Å] V [Å³] 0 0.13094.1830(1) 73.2 6.48 0.2637 4.2114(2) 74.7 12.77 0.3985 4.2212(2) 75.224.77 0.5352 4.2893(4) 78.9

[0129] For the BaCeO₃ sample (i.e. containing 43.05% Ce b.w., x=1): thefollowing lattice parameters were obtained:

[0130] Orthorhombic phase

a=6.204(1) Å; b=6.235(1) Å; c=8.760(1) Å; V=338.9 Å³;

[0131] Cubic phase

a=4.3924(2) Å; V=84.7 A³.

[0132] It can be observed that the substitution of zirconium with ceriuminvolves, as frequently observed for perovskites, a deviation from theideal classical cubic structure because the cerium ion (Ce⁺⁴) inoctahedral co-ordination has a radius (128 pm) greater than that ofzirconium (Zr⁺⁴) with the same co-ordination (86 pm). This substitutionthus results in a certain distortion, and as a consequence, theformation of a phase which is no longer cubic but orthorhombic. For thisreason, in FIG. 9 the cell volume (V), rather than (a), is-reported as afunction of x.

[0133] In addition, various phases were identified by the search-matchmethod (JCPDS data base), while the phase composition and the cellparameters were determined with great accuracy by full profile fittingrefinement. In this search the Rietvield method was used (16), followingthe Hill&Howard procedure (17) and the WYRIET program. The necessarystructural data were taken from ICDS (18).

[0134] Catalytic activity tests were performed using 0.4 g of catalyst(with surface area not optimised) mixed with 0.8 g of quartz (140-200mesh) and placed inside a microreactor of 8 mm i.d. On top of thecatalytic bed a 1.2 cm layer of quartz particles (20-30 mesh) is placed.A gas flow, constant in composition and rate, was used:

[0135] Inlet gas composition

[0136] Methane: 0.8%; Oxygen: 95.7%; Nitrogen: 3.5%

[0137] Flow rate

[0138] 400 Ncc/min

[0139]FIG. 10 gives the temperatures at which 20% conversion is reachedin runs performed at increasing temperature. The data were obtainedusing samples not optimised in surface area.

EXAMPLE 7 Characterisation of the BaZr_(1−x)Rh_(x)O₃ Samples

[0140] Table 15 shows the cell parameters as a function of x. These datashow a constant and regular decrease of the cell parameters. As in thepalladium containing systems, these rhodium containing systems show thatrhodium is present in a high oxidation state, probably as Rh⁺⁴. TABLE 15Rh x wt. % a (Å) V (Å³) 0 — 4.1815   73.11 0.0674 2.5 4.1816(1) 73.120.0674 2.5 4.1813(1) 73.10 0.1351 5 4.1803(1) 73.05 0.2718 10 4.1640(1)72.20 0.5101 18.58 4.1377(2) 70.84

EXAMPLE 8 Characterisation of the Ba_(1−x)La_(2/3x)ZrO₃ Samples

[0141] Table 16 shows the values of the cell parameter a and the cellvolume V for the sample with x=0.25 TABLE 16 x a (Å) V (Å³) 0.254.1730(1) 72.67

[0142] The smaller steric requirements of La⁺³ compared to Ba⁺² leads toa small decrease in the cell volume.

[0143] Thermal Stability Tests

[0144] Numerous tests of thermal stability were performed at variousstages of the preparation and for different sample compositions: on thesolution and on dried samples at various temperatures. Tests wereperformed with a differential scanning calorimeter DSC Mettler 800,using stainless steel closed crucibles, from room temperature up to 780°C. in air, and with a heating rate of 10° C. All the samples showed agood stability and may therefore be handled safely. This is in contrastto current literature warnings that, for example, BaO₂ should not bemixed with water: such restrictions need not apply for water solutionsof citric acid, which allow for the complexation of barium with agradual and controlled release of oxygen in the form of small gasbubbles, thereby ensuring safe working.

References

[0145] 1) ‘Concise Encyclopaedia of Advanced Ceramic Materials’ 349-351,Ed. R. J. Brook, Pergamon Press;

[0146] 2) J. E. Huheey, ‘Inorganic chemistry: Principle of structure andreactivity (3^(rd edition))’, University of Maryland—Harper CollinsPublishers;

[0147] 3) J. G. Mc Carty, M. Gusman, D. M. Lowe, D. L. Hildenbrand, K.N. Lau, Catalysis Today 47 (1999) 5-17;

[0148] 4) A. K. Datye, J. Bravo, T. R. Nelson, P. Atanasova, M.Lyubovsky, L. Pfefferle, Applied Catalysis 198 (2000) 179-196;

[0149] 5) K. Fujimoto, F. H. Ribeiro, M. Avalos-Borja, E. Iglesia,Journal of Catalysis 179 (1998) 431;

[0150] 6) Italian Patent Application MI 96 A 002011;

[0151] 7) Christian Marcilly, Bernard Delmon, C. R. Acad. Paris, t. 268(18-5-1969) Série C 1795;

[0152] 8) Philippe Courty, Bernard Delmon, C. R. Acad. Paris, t. 268(18-5-1969) Série C 1874;

[0153] 9) Philippe Courty, Bernard Delmon, Powder Technology 7 21(1973);

[0154] 10) Bernard Delmon, Christian Marcilly, Andre Sugier, PhilippeCourty, Brevet n ^(o) 1604707 déposé le 2 juillet 1968;

[0155] 11) Philippe Courty et al, DE 1933331 Jan. 29, 1970

[0156] 12) André Sugier, Bernard Delmon, Brevet n^(o) 7013305 déposé le13 avril 1970;

[0157] 13) PCT/EP91/02404;

[0158] 14) PCT/EP90/01823;

[0159] 15) Italian Patent Application MI 93 A 002704;

[0160] 16) R. A. Young, ‘The Rietveld Method’, International Union ofCrystallography, Oxford University Press 1993;

[0161] 17) R. J. Hill, C. J. Howard, J. Appl. Ciyst. 20 (1987) 467;

[0162] 18) ‘Inorganic Crystal Structure Database’, Karlsruhe GmelinInstitut für Anorganische Chemie und Fachinformationszentrum FIZKarlsruhe (1997).

1. Materials with a perovskite structure in the form of solid solutionsof general formula: AzZr_(1−x)B_(x)O₃ where A is Ba or a rare earthelement, B is Pt, Ir, Pd, Rh or Ce, Z is 1 when A is Ba and ⅔ when A isa rare earth, X is in the range 0.01-0.8.
 2. Materials according toclaim 1 in which A is Ba or La.
 3. Materials according to claim 1 inwhich B is Pd, Rh, or Ce.
 4. Materials according to claim 3, taken fromB_(a)Z_(r1x)Pd_(x)O₃, BaZ_(r1−x)C_(ex)O₃, BaZ_(r1−)xRh_(x)O₃.
 5. Use ofthe materials of claim 1 as catalysts.
 6. Use according to claim 5 ascatalysts for the catalytic combustion of methane for power application.7. Use according to claim 5 for the catalytic partial oxidation ofmethane to syngas.
 8. Use according to claim 5 for catalysts forcatalytic mufflers for motor vehicles.
 9. Use according to claim 5 forcatalysts for the elimination of VOC.
 10. Use according to claim 5 forthe oxidation of light alkanes to the corresponding olefins.